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Risk Priority Number (RPN)
Numerical value assigned to three outcomes
1. Likelihood of occurrence (OCCUR/O)
2. Likelihood of detection (DETEC/D)
3. Severity (SEV/S)
Numerical value from 1-10 assigned
RPN=O x S x D
Max = 1000
Rank
Occurrence (O)
Severity (S)
Detectability (D)
Qualitative Frequency
Qualitative
Categorization Qualitative
Estimated
Probability
of going
undetected
(%)
1
Failure
unlikely
1/10,000
No effect
Very easy to
detect; QA
checks already
in place
0.01
2
2/10,000
Inconvenience Inconvenience
0.2
3
Relatively
few failures
5/10,000
Easy to detect,
could be missed
without double
check, human
error (e.g.
transcription
error)
0.5
4
1/1,000
Minor
dosim tric rror
Suboptimal plan
or treatment
1.0
5
<0.2%
Limited toxicity
or under-dose
Wrong dose,
dose
distribution,
location or
volume
Moderate, a
“lucky catch”
2.0
6
Occasional
failures
<0.5%
Very difficult to
detect
5.0
7
<1%
Potentially
serious toxicity
or under-dose
10
8
Repeated
failures
<2%
15
9
<5%
Possible very
serious toxicity
Very wrong
dose, dose
distribution,
location or
volume
Almost
impossible to
detect; no QA
in place
20
10
Failures
inevitable
>5%
Catastrophic
>20